Behavior Genetics

In modern societies, individual differences in completed fertility are linked with genotypic differences between individuals. Explaining the heritability of completed fertility has been inconclusive, with alternative explanations centering on family formation timing, pursuit of education, or other psychological traits. We use the twin subsample from the Midlife Development in the United States study and the TwinsUK study to examine these issues. In total, 2606 adult twin pairs reported on their completed fertility, age at first birth and marriage, level of education, Big Five personality traits, and cognitive ability. Quantitative genetic Cholesky models were used to partition the variance in completed fertility into genetic and environmental variance that is shared with other phenotypes and residual variance. Genetic influences on completed fertility are strongly related to family formation timing and less strongly, but significantly, with psychological traits. Multivariate models indicate that family formation, demographic, and psychological phenotypes leave no residual genetic variance in completed fertility in either dataset. Results are largely consistent across U.S. and U.K. sociocultural contexts.

House mouse lines bidirectionally selected for nest-building behavior show a correlation between number of AVP cells in the suprachiasmatic nuclei (SCN), the master circadian clock in mammals, and level of nest-building behavior as well as a correlation between wheel-running activity and SCN AVP content. Similar genetic correlations between wheel-running activity and nest-building behavior have been reported in house mouse lines selected for increased voluntary wheel-running behavior. These similarities in genetic correlation structure in independently selected mouse lines allowed us to test whether AVP in the SCN and wheel running activity are truly correlated traits under identical testing procedures. In the mouse lines selected for voluntary wheel-running, no difference was found between the lines selected for high levels of voluntary wheel-running and randomly-bred control lines in the number of AVP immunoreactive neurons in the SCN (F 1,6 = 0.09, NS; replicate line effect: F 1,22 = 0.05, NS). This finding was confirmed at the level of individual variation, which revealed no relationship between number of AVP neurons in the SCN and total daily activity (R = −0.086, NS, n = 24), or circadian organization (i.e., the chi-squared periodogram waveform amplitude; R = −0.071, NS). Therefore our data do not support the hypothesis that differences in activity level and the circadian expression of activity in young adult mice are related to differences in the number of AVP-immunoreactive cells in the SCN.

Drosophila melanogaster lines previously selected for fast and slow return of female receptivity were subjected to a chromosome substitution analysis. Chromosomal effects on direct response to selection were distinctively different between selection lines derived from two different base populations. All three chromosomes tested affect the trait in the JEFFERS selection lines. In contrast, only chromosome II was found to have a main effect in the COMP selection lines. Significant interactions between chromosome II and the other chromosomes were also found in both of the selection lines. All of the components of virgin fly mating behavior measured were affected by chromosome II.

Composite general cognitive ability scores were computed for members of 843 families from the Americans of European Ancestry sample in the Hawaii Family Study of Cognition (HFSC) and were analyzed for linearity of offspring-parent resemblance. Regressions of midoffspring on midparent values using various subgroupings of the data set, as well as a test for the significance of the quadratic regression obtained from an analysis of the entire sample, provided little or no evidence for nonlinear offspring-parent resemblance. These results differ from those recently reported by Reed and Rich [(1982), Behav. Genet.12:535–542], possibly because all family members tested in the HFSC are within the normal range of mental ability, whereas the Reed and Rich sample consists of families having a history of mental retardation.

In female choice experiments,Drosophila melanogaster males from a selected photonegative strain show a highly significant mating advantage over males from a photopositive strain. While photonegative behavior is sex linked in this species, the X chromosome is not involved in the mating advantage seen for this strain. The degree of the photonegative male advantage changes when tests are conducted with females mutant for olfaction and auditory perception but not when females are blind.

In the holometabolous insect Drosophila melanogaster, genetic, physiological and anatomical aspects of olfaction are well known in the adult stage, while larval stages olfactory behavior has received some attention it has been less studied than its adult counterpart. Most of these studies focus on olfactory receptor (Or) genes that produce peripheral odor recognition. In this paper, through a loss-of-function screen using P-element inserted lines and also by means of expression analyses of larval olfaction candidate genes, we extended the uncovering of the genetic underpinnings of D. melanogaster larval olfactory behavior by demonstrating that larval olfactory behavior is, in addition to Or genes, orchestrated by numerous genes with diverse functions. Also, our results point out that the genetic architecture of olfactory behavior in D. melanogaster presents a dynamic and changing organization across environments and ontogeny.

Fingerprints and handedness were measured in 2169 subjects. Left-handers were more likely to have an increased number of arches and radial loops, while having fewer whorls and lower pattern intensity than right-handers. Analyzing by hand and digit, the differences were most marked on the left hand, which showed significant differences on four of the five digits which were related to handedness. On the right-hand, handedness was associated with fingerprint patterns only on digit IV (ring finger). This finding suggests a genetic mechanism in the development of handedness and may provide a means of separating pathological from natural left-handers.

X-linked genetic differences between male and females have been posited to cause greater variance in cognitive ability in males. Males with only one X chromosome tend to express the genes on the X chromosome more fully than females, who express an “average” of their two X chromosomes due to X-inactivation. Greater variability in expression of genes on the X chromosome could account for greater variability in male cognitive ability. This would affect both the high and low ends of the cognitive ability distribution, but the possibility of high-end impact has drawn the most attention and controversy. The objective of this paper was to outline a method to test for empirical evidence that greater X-chromosomal variation in males is associated with greater variation in cognitive ability in males at the high end of the distribution. The method utilizes exogenous variation in the maternal X chromosome of twins to test the effect of sex on within-pair variation. We applied this method to g composite test scores at age 10 using data from the Twins Early Development Study. Twin-pair zygosity was used as an instrument reflecting whether twins had different maternal X chromosomes. We estimated differences in the association between variation in the maternal X chromosome in males and females and within-pair variation in test scores using a difference in differences specification of a linear regression model. We found no evidence supporting the proposition that the “averaging” effect of X-inactivation in females resulted in greater variation in male cognitive ability. There was evidence of differential selection for cognitive ability in the sample, however, indicating that the value of our study was primarily to introduce a novel method of addressing the question.